At the present time, gel-casting has shown the ability to fabricate complex shaped articles, but because of high slurry viscosities at high solids contents, de-airing is insufficient and defects are introduced into the parts. This has been observed in silicon nitride materials.
As the solids contents of a slurry increases, the viscosity also increases. This behavior is more a problem with small or fine particle sizes. For fabrication techniques, such as gel-casting, it is desirable to process at high solids contents to obtain parts with as high of green densities as possible, Green densities of .gtoreq.48% are considered a minimum necessary for adequate sintering. With fine powders used in current advanced ceramics (&lt;1 .mu.m diameter) and using conventional dispersion techniques, such as ball milling, the slurries typically have very high viscosities (&gt;100 cP) and are difficult to de-air at solids loadings &gt;40 volume %. Consequently, these slurries contain large numbers of bubbles that are retained in the gel-cast part as voids and end up as defects or flaws in the final piece. At solids contents approaching 50 volume %, the slurries become unpourable and thus cannot even be gel-cast.